Zoom lens

ABSTRACT

In a zoom lens ( 30 ) comprising, from the object side to the image side, a front lens group ( 12 ) and a controllable lens group ( 24 ), the controllable lens group comprises two lens elements ( 25, 26 ) having different dispersions and being movable with respect to each other so as to perform a focusing action. One of the lens elements corrects for the dispersion of the zoom lens. Preferably, the zoom lens comprises at least one folding mirror.

The invention relates to a zoom lens having at least a front lens groupand a controllable lens group.

The invention also relates to a camera comprising such a zoom lens andto a handheld device comprising such a camera.

A conventional zoom lens comprises a number of solid lens elements madeof a transparent material like glass or a transparent plastic. Theselens elements are grouped in a front lens group at the object side, arear lens group at the image side, and a controllable lens group betweenthe front group and the rear group. Each of these groups may consist ofone or more lens elements. Lens elements of the controllable lens aremovable for performing zooming. Zooming is understood to mean changingthe image scale, i.e. selecting the size of the object scene that isimaged, by changing the focal distance of the zoom lens. The maximumsettings of the zoom lens are Tele configuration, wherein a smallportion of an object scene is imaged, and wide configuration, wherein alarger portion of an object scene is imaged. By moving lens elements ofthe controllable lens group the zoom lens can be set between these twoextreme configurations and configurations therebetween.

Currently miniature cameras are incorporated into handheld apparatuses,like mobile phones and thus the need arises for a zoom lens for such acamera. The built-in height of a zoom lens for a miniature camera shouldbe adapted to the space available in a handheld apparatus. The built-inheight of a camera to incorporated into a mobile phone is the distancebetween the front lens element and the rear of the image sensor of thecamera, which sensor converts the received image into electricalsignals. This is because, the front lens element should be accommodatedin the front surface of the apparatus. The optical axis of the zoom lensthus extends in the depth direction of the apparatus. A practicalrequirement for a mobile phone or other handheld apparatus is that itsdepth, or thickness, is small. The built-in height of a camera with azoom lens is mainly determined by the space that is needed to displacethe controllable lens group.

In addition to a zooming action, a focusing action should be carried outin a zoom lens. Focusing is understood to mean keeping the selectedobject scene in focus for every configuration of the zoom lens system.Conventionally the focusing action can be performed by moving either thefront lens group or the image sensor up and down along the axis of thezoom lens. This requires additional space and increases the length ofthe zoom lens and thus the built-in height of the camera.

If in a lens system, such as a miniature zoom lens, the number of lenselements should be as small as possible, a holographic grating may beincluded, for example in the controllable lens group in the case of azoom lens, to correct for the dispersion of the lens elements. Thediffraction angle of such a grating is different for the differentwavelengths of the natural light incident on it. This wavelengthdependency can be used to substantially correct for thewavelength-dependent behavior of the lens elements of the zoom lens. Ifno diffraction grating is used, additional elements will have to be usedfor correction of the dispersion of the lens elements. However, thediffraction efficiency of the grating is optimum for one wavelength onlyand lesser for other wavelengths. This means that not all of the lightof said other wavelengths is deflected in the required direction, whichresults in additional, ghost, blurred images being formed on the imagesensor by said other wavelengths. The contrast of the image on the imagesensor will thus be impaired.

It is an object of the invention to provide a zoom lens which has asmall built-in height, thus is suitable for a miniature camera, and iswell corrected for dispersion. This zoom lens is characterized in thatthe controllable lens group comprises two lens elements which are madeof different lens materials and are movable with respect to each other.

Distributing the lens function of the controllable lens over two lenselements allows this lens to perform two additional functions. Thedispersion can be corrected by the use of lens materials with differentrefractive indices. Moving the lens elements with respect to each othermakes focusing possible, so that no extra space is required for movingeither the front lens group or the image sensor. Zooming is performed bymoving the entire controllable lens group.

A preferred embodiment of the zoom lens having a lens stop ischaracterized in that the lens stop is arranged near the first lenselement of the controllable lens group and is movable together with thislens group.

The stop in a lens system is the smallest opening, formed by adiaphragm, in the system. This stop restricts the diameter of theimaging beam and prevents stray radiation or radiation from unwantedreflections from being introduced intro the imaging beam and causing areduction of the contrast in the image. Arranging the stop near thecontrollable, i.e. movable, lens group, and moving the stopsimultaneously with this lens group allows a restriction of the diameterof the lens elements of this group to approximately that of the stopopening. The use of such small lens elements reduces the chance of falselight being coupled into the lens system.

The position of the stop in the zoom lens system is also of greatimportance for an essential feature that allows a substantial reductionof the built-in height of the zoom lens. An embodiment of the zoom lenswhich includes this feature is characterized in that it comprises atleast a folding mirror arranged between the front lens group and thecontrollable lens group.

The special design of the zoom lens having only a front lens groupcomprising a single lens element and the controllable lens allows thecreation of sufficient space between the front group and thecontrollable group. The embodiment with the folding mirror renders itpossible to arrange the main portion of a camera with the zoom lensparallel to the main surface of a device in which the camera is to beincorporated.

The design of conventional zoom lenses does not allow introduction of afolding mirror; there is not enough space between the lens elements toarrange a folding mirror.

It is noted that U.S. Pat. No. 6,339,508 discloses a zoom lens for aminiature camera to be incorporated into a mobile phone, which zoom lensincludes a folding mirror between a front lens element and a movablelens group. The group comprises a relatively large number of lenselements. In this zoom lens, focusing is performed by moving the frontlens element or the rear lens element, for which movement space has tobe reserved.

A further reduction in size is obtained in an embodiment which ischaracterized in that it comprises a second folding mirror arrangedbehind the controllable lens group.

In this way the track length of the zoom lens can be reduced withoutsubstantially increasing its dimension in the direction perpendicular tothe length direction.

U.S. Pat. No. 4,249,798 discloses a zoom lens for a pocket camera, whichzoom lens comprises a first folding mirror between a front lens groupand a movable lens group and a second folding mirror between the movablelens group and the rear lens group. The front lens group and the rearlens group each comprise two lens elements, and the movable lens groupcomprises four lens elements. The lens stop is arranged between thethird and the fourth lens element of the movable lens group. In thiszoom lens again focusing is not performed in that moving elements of themovable lens group are moved with respect to each other.

An embodiment of the zoom lens having a folding mirror at the objectside portion may be further characterized in that this folding mirror isintegrated with the front lens group.

In this way the number of elements of the zoom lens is reduced by one,which reduces the manufacturing cost.

Another embodiment of the zoom lens having two folding mirrors ischaracterized in that the first folding mirror is arranged such that theangle between the normal to its reflective surface and the optical axisis greater than 45°.

The image sensor may then be arranged closer to the optical axis of thezoom lens so that the total size of the camera can be further reduced.

As a camera wherein the zoom lens is incorporated is distinguished fromconventional cameras by features provided by the invention, such acamera forms part of the invention.

As the incorporation of such a camera into a handheld apparatus providessuch an apparatus with a zooming function, such an apparatus also formspart of the invention.

These and other aspects of the invention are apparent from and will beelucidated, by way of non-limitative example, with reference to theembodiments described hereinafter. In the drawings:

FIGS. 1 a and 1 b show a design principle of a zoom lens for a miniaturecamera, with ray trace plots for the Tele configuration and the wideconfiguration, respectively, of this zoom lens;

FIGS. 2 a and 2 b show an embodiment of a zoom lens according to theinvention and ray trace plots for the Tele configuration and the wideconfiguration, respectively, of this zoom lens;

FIGS. 3 a and 3 b show an embodiment of this zoom lens, which isprovided with a first folding mirror and similar ray trace plots;

FIGS. 4 a and 4 b show an embodiment of this zoom lens with a first anda second folding mirror and similar ray trace plots;

FIGS. 5 a and 5 b show another embodiment of the zoom lens with twofolding mirrors;

FIG. 6 shows a zoom lens having an integrated folding mirror and frontlens element;

FIG. 7 shows a mobile phone provided with a camera, which includes azoom lens according to the invention, and

FIG. 8 shows a laptop computer provided with a camera, which includes azoom lens according to the invention.

In these Figs, the same reference numbers denote the same elements.

FIGS. 1 a and 1 b show a basic design of a zoom lens 10 for a miniaturecamera 1. This camera has a CCD or CMOS image sensor 18 and associatedprocessing electronics (not shown). The camera should be so small thatit can be incorporated into a mobile phone or another handheldapparatus. The requirements to be set for such a zoom lens are mainlydetermined by features of the image sensor, such as its size andresolution and the required zoom factor. In practice the sensor has adiagonal dimension of, for example, 3.36 mm, and theresolution-determining pixel size, is for example, between 5.6 μm and4.2 μm. A zoom factor of 2 is sufficient. It has been observed that forthis application a zoom lens can be designed which has only two lenselements for the image formation. These lenses are a front lens element12 forming the front lens group and a movable lens element 14 formingthe controllable lens group 12. A lens stop 16 is formed by a diaphragmthat is arranged near lens element 14 at the side of the front lenselement 12. This diaphragm restricts the diameter of the imaging beamand prevents stray radiation or radiation from unwanted reflections frombeing introduced into the imaging beam and causing a reduction of thecontrast in the image formed on the sensor. Such a diaphragm ensuresthat the beam diameters are the same for all imaging beam portions. Theoptical track of the zoom lens, i.e. the distance between the frontsurface of the front lens element and the front surface of the imagesensor (image plane), may be, for example, as small as 14 mm.

Zooming, i.e. setting the image size, is performed by displacing thelens element 14 together with the diaphragm 16 along the optical axisOO′, thereby changing the focal distance of the zoom lens.

FIG. 1 a shows a ray trace plot for a Tele configuration of the zoomlens wherein a small object of the viewed scene is imaged. FIG. 1 bshows a ray trace plot for a wide configuration of the zoom lens whereina large object of the scene is imaged.

In order to keep the image focused on the image sensor for each zoomconfiguration, a focusing action is needed in addition to a zoomingaction. Conventionally, such a focusing action is performed bydisplacing the front lens group or the image sensor along the opticalaxis of the zoom lens. Space should be reserved for such a displacement,whereby the camera provided with such a zoom lens would be enlarged.

Furthermore, the lens elements of the zoom lens suffer from dispersion,i.e. components of the imaging beam b having different wavelengths arerefracted by these lens elements in different ways. To limit the numberof lens element to a minimum, a holographic grating may be incorporatedin the zoom lens, which grating diffracts components of the imaging beamhaving different wavelengths in different ways. The grating and the zoomlens can be designed such that the dispersion of the lens elements canbe corrected by the wavelength-dependent diffraction of the grating.However, the efficiency of the grating, i.e. the ratio of the amount ofradiation diffracted in a required diffraction order to the amount ofradiation incident on the grating, is wavelength-dependent. The gratingshows an optimum diffraction efficiency for only one wavelength. Forother wavelengths a part of the radiation is diffracted in unwantedorders, which results in ghost and blurred images on the image sensorand thus in a decrease in the contrast of the image formed on the imagesensor.

To avoid the decrease in contrast and to allow focusing withoutdisplacing the front lens group or the image sensor, the zoom lens ofthe invention comprises a controllable lens group, which is composed oftwo lens elements. FIGS. 2 a and 2 b show a first embodiment of such azoom lens 20. The new controllable lens group is denoted by referencenumber 24 and its lens elements by 25 and 26. Lens element 25 may be apositive element having two convex surfaces, and lens element 26 may bea negative lens having two concave surfaces. Lens elements 25 and 26 aremade of different lens materials having different dispersions, or Abbenumbers, which allows one of these elements to correct for thedispersion of the other lens element. For example, one of the lenselements 25 and 26 may be made of polycarbonate (PC) whilst the otherlens and the front lens element may be made of polymethyl methacrylate(PMMA). Both materials are well-known optical materials. Alternativetransparent plastic materials, like cyclic olefin polymer (COC), may beused for the lens elements.

Lens elements 25 and 26 are driven by separate motor drives, known perse and not shown in FIGS. 1 a and 1 b, so that the lens elements can bemoved independently. Focusing is performed in that the lens elements 25and 26 are moved relative to each other. Zooming is performed in thatthe lens elements are moved simultaneously in the same direction andover the same distance to and from the front lens element, as shown inFIGS. 2 a and 2 b, which show the Tele and the wide configuration of thezoom lens, respectively.

As is shown in FIGS. 2 a and 2 b, a lens stop 16 is arranged at lenselement 25, and during a zooming action the stop moves simultaneouslywith and in the same direction and over the same distance as lenselements 25 and 26. As the diameter of a lens element near the lens stopmay be approximately the same as the diameter of the stop aperture, thesize of the lens elements can be kept small so that no or a negligibleamount of false light will reach the image sensor.

The zoom lens design shown in FIGS. 1 a–2 b makes it possible tointroduce a folding mirror into the zoom lens. In a conventional zoomlens design there is insufficient space available between the lenselements for such a mirror.

FIGS. 3 a and 3 b show an embodiment of a zoom lens 30 having a foldingmirror 32 arranged between the front lens element 12 and the first lenselement of the controllable lens group 24. The imaging beam b from theobject scene is incident perpendicularly on the front lens element 12.After having passed this element, the beam is reflected by the mirror 32at an angle of 90°, i.e. in the horizontal direction, if this mirror isarranged at an angle of 45° with respect to the chief ray of the beam b.The other elements: the lens elements 25 and 26 and the image sensor 18are arranged in the horizontal direction. This horizontal direction isparallel to the front surface of the apparatus, wherein the camera, i.e.the zoom lens and the images sensor, should be built in. In this way thebuilt-in height h_(c) of the camera is reduced to the sum of the heightof the mirror 32 and the thickness of the front lens element 12.

Including a folding mirror in the zoom lens allows a reduction of thebuilt-in height of the camera to a substantially greater extent than ispossible by including a controllable lens group having two independentlymovable lens elements. However, including such a lens group in a zoomlens, which is already provided with a folding mirror, allows the use ofthe latter possibility to reduce the built-in height of the camerafurther and make this height constant.

As is clear from FIG. 3 a, which shows a ray trace plot for the Teleconfiguration of the zoom lens, the lens stop 16 in this configurationis positioned very close to the folding mirror 32. FIG. 3 b shows a raytrace plot of the wide configuration of the zoom lens.

Providing the zoom lens with a second folding mirror can further reducethe total size of the zoom lens. FIGS. 4 a and 4 b show an embodiment ofa zoom lens with such a second mirror and ray trace plot for the Teleconfiguration and the wide configuration, respectively. The secondfolding mirror 42 is arranged between the movable lens element 26 andthe image sensor 18. This mirror reflects the imaging beam b coming fromlens element 26 at an angle of 90°, i.e. in the vertical direction, ifit is arranged at 45° with respect to the chief ray of the beam, so thatthe image sensor can be arranged in this direction. In this way thedimension of the zoom lens, and thus of the camera, in the horizontaldirection can be reduced without substantially enlarging the dimensionin the vertical direction. This dimension, i.e. the built-in height, isnow the distance between the front surface of the front lens element andthe front surface of the image sensor. This height may be as small as3.7 mm. Some additional height, for example 1.5 mm may be required forthe image sensor and its packaging.

The folding mirror 42, which reflects the beam downwards, may bereplaced by a folding mirror arranged at an angle of 90° with respect tomirror 42 so that the beam is reflected upwards. The image sensor 18 canbe arranged at approximately the same height as front lens element 12 inthis case.

The preference for a second folding mirror is determined by thethickness of the image sensor. If this sensor, including its packagingis small enough, or if the sensor can be sunk into its printed circuitboard (PCB) so that the PCB can be arranged closer to the lens elements,no second mirror is needed.

Adapting the tilt of the first folding mirror can further reduce thebuilt-in height of the zoom lens and camera. This is illustrated inFIGS. 5 a and 5 b, which show ray trace plots for the Tele configurationand the wide configuration, respectively, of such a modified embodiment50 of the zoom lens. The first folding mirror 12 is arranged such thatthe angle of incidence θ of the imaging beam b on this mirror is largerthan 45°. The beam is reflected slightly upwards from the horizontaldirection and the second folding mirror 42 and image sensor are shiftedupwards so that they are more in line with the font lens element 12 andthe first mirror 32. Although FIGS. 5 a and 5 b shows a zoom lens with asingle movable lens element 14, it will be clear that the first foldingmirror with the adapted tilt can also be used in a zoom lens as shown inFIGS. 2 a–4 b.

As is shown in FIG. 6, the first folding mirror may be integrated withthe front lens element into a mirror lens element 62. This element has areflective flat base surface 64 arranged at an angle of, for example,45° with respect to the chief ray of the beam b, and two curved surfaces66 and 68 which form the refractive surfaces of the lens element. Inthis way the number of elements of the zoom lens and thus itsmanufacturing cost can be reduced. At least one of the curved surfacesof element 62 may be an aspherical surface, as is the case for the otherlens elements of the zoom lens embodiments described above.

A practical embodiment of the new zoom lens having two movable lenselements and shown in FIGS. 2 a and 2 b has the followingcharacteristics:

Tele wide Focal length 7.2 mm 3.6 mm F/number 5.9 4.0 Angle of viewdiagonal 26.8° 56°

This zoom lens is suitable for cooperation with a CCD or CMOS imagesensor having a diagonal dimension of 3.36 mm and comprising 640×480pixels, so that the pixel pitch is 4.2 μm.

FIG. 7 shows an example of a handheld apparatus in which the zoom lensof the invention can be used. The apparatus is a mobile phone 70 shownin front view in FIG. 7. The mobile phone has a microphone 72 whichinputs the user's voice as data, a loudspeaker 74 which outputs thevoice of a communicating partner, and an antenna 76 which transmits andreceives the communicating waves. The mobile phone further comprises aninput dial 78 by means of which the user inputs data, such as a phonenumber to be dialled, and a display 80, for example a liquid crystaldisplay panel. This panel may be used to display a picture of thecommunicating partner or of the user, or to display data and graphics. Adata processing unit (not shown) is included in the mobile phone forprocessing the input data and the received data.

The phone 70 is provided with a miniature camera 82 comprising a zoomlens as described above for photographing a scene, graphics, or data tobe communicated to the partner or the user. Of this camera only theentrance surface 84 of the front lens element 12 of the zoom lens isvisible. The other elements of the camera, i.e. the movable lenselements and the image sensor, may be arranged along a lineperpendicular to the front surface of the phone, i.e. in the directionperpendicular to the plane of drawing of FIG. 7, if the dimension of thephone in this direction is large enough. Preferably, the zoom lenscomprises at least one folding mirror. Then at least the two movablelens elements are arranged along a line parallel to the front surface ofthe phone, which may then be relatively thin.

The front lens element of the zoom lens may also be arranged in the rearsurface of the mobile phone. If the mobile phone is provided with anenvelope or cover portion, the zoom lens may be arranged in thisenvelope, and the front lens element may be arranged in a main surfaceof this envelope.

Another handheld apparatus in which the invention may be implemented isa personal digital assistant (PDA) provided with a miniature camera.Such a camera with a zoom lens as described above may be arranged in thePDA in the same way as described for the mobile phone.

FIG. 8 shows a laptop computer (notebook) as an example of a portableapparatus in which the invention may be implemented. The laptop computer90 comprises a base portion 92 in which a keyboard 95 and the processorunit are incorporated. A cover portion 96, which can be rotated withrespect to the base portion, includes a display 98 and a miniaturecamera 100. Such a camera provided with a zoom lens as described abovemay be arranged in the laptop in the same way as described for themobile phone.

The invention has been described with reference to a zoom lenscomprising only a front element and a controllable lens group, whichzoom lens is intended to be used in a miniature camera having a smallimage sensor with a moderate pixel pitch, or resolution. A zoom lens fora camera having another type of image sensor may have a rear lenselement and a front and/or rear lens group having more than one lenselement. The invention may also be used in such a type of zoom lens.Furthermore, the invention can be used not only in a miniature camerafor a handheld apparatus, like a mobile phone, a digital personalassistant, a pocket computer and an electronic toy, or for a portableapparatus, but also in other types of built-in cameras. The inventionmay also be used in non-built-in cameras, like cameras for desktopcomputers, cameras for intercom systems, and pocket-sized and other-sizecameras, for example digital cameras. The camera may be a still picture(photo) camera or a video camera. It is irrelevant for the inventionwhether the camera uses a film or an electronic sensor, for example aCCD sensor or CMOS sensor.

1. A zoom lens having at least a front lens group and a controllablelens group, characterized in that the controllable lens group comprisestwo lens elements which are made of different lens materials and aremovable with respect to each other for focusing the zoom lens withoutdisplacing the front lens group, and further characterized in that thezoom lens comprises a folding mirror arranged between the front lensgroup and the controllable lens group.
 2. A zoom lens as claimed inclaim 1 and having a lens stop, characterized in that the lens stop isarranged near the first lens element of the controllable lens group andis movable together with this lens group.
 3. A zoom lens as claimed inclaim 1, characterized in that it comprises a second folding mirror,which is arranged behind the controllable lens group.
 4. A zoom lens asclaimed in claim 3, characterized in that the folding mirror is arrangedsuch that the angle between the normal to its reflective surface and theoptical axis of the front lens group is greater than 45°.
 5. A zoom lensas claimed in claim 1, characterized in that the folding mirror isintegrate with the front lens group.